Morphology and porosity of the spines of the sea urchin Heterocentrotus mamillatus and their implications on the mechanical performance
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Spines of the slate pencil sea urchin Heterocentrotus mamillatus Linnaeus, 1758, are in focus of biomimetic research as they feature a “graceful” failure behaviour under uniaxial compression dissipating energy and resisting high loads even after high strain. This study elucidates and quantifies the organization of calcitic trabeculae and pores in large primary spines of the slate pencil urchin H. mamillatus by image analysis from scanning electron microscopy, X-ray micro-computed tomography (µCT) and gravimetry. This study delivers a detailed distribution of porosities within the whole spine and shows that parts of the spines have a much higher porosity then hitherto thought. The central part (medulla) of the high-magnesium calcitic stereom of H. mamillatus spines has a porosity range of 75% to nearly 90%. From this innermost structure, more than 200 radially aligned, but often sinuous trabeculae extend to the spine rim. The structure of this complicated meshwork (radiating layer) is best seen in basal cross sections and was confirmed by µCT scans. The radiating layer has a porosity range from 40–70% and is irregularly separated by the dense growth layers (15–35% porosity). Growth layers were classified in proximal and distal growth layers with numbers ranging within a single animal between 3–14 and 2–7, respectively. These growth layers are characteristic for H. mamillatus spines and play a major role in their remarkable mechanical properties. The porosity of the spine increases from base to tip. Biological and mechanical implications of the variations are discussed.
KeywordsMorphology Echinoids Sea urchin spines Porosity µCT scans
The authors gratefully thank the German Research Foundation (DFG—Deutsche Forschungsgemeinschaft) for funding this work within the framework of the Collaborative Research Centre (SFB/Transregio) 141 “Biological Design and Integrative Structures” project B01. We also thank Barbara Maier and Simone Schafflick in the workshop for their support. The work of an anonymous reviewer is kindly appreciated.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
Statements on the welfare of animals
The sea urchins were purchased dead from a fossils collector, were not killed for the purpose of this study and are not listed as endangered species.
This article does not contain any studies with human participants performed by any of the authors.
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